Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2001. / Includes bibliographical references (leaves 122-125). / Nonlinear dynamic physical systems exhibit a rich variety of behaviors. In many cases, the system response is unstable, and the behavior may become unpredictable. Since an unstable or unpredictable response is usually undesirable in engineering practice, the stability characterization of a system's behavior becomes essential. In this work, a numerical procedure to characterize the dynamic stability of continuous solid media, discretized using finite element methods, is proposed. The procedure is based on the calculation of the maximum Lyapunov characteristic exponent (LCE), which provides information about the asymptotic stability of the system response. The LCE is a measure of the average divergence or convergence of nearby trajectories in the system phase space, and a positive LCE indicates that the system asymptotic behavior is chaotic, or, in other words, asymptotically dynamically unstable. In addition, a local temporal stability indicator is proposed to reveal the presence of local dynamic instabilities in the response. Using the local stability indicator, dynamic instabilities can be captured shortly after they occur in a numerical calculation. The indicator can be obtained from the successive approximations of the response LCE calculated at each discretized time step. Both procedures can also be applied to fluid-structure interaction problems in which the analysis focuses on the behavior of the structural part. / (cont.) The response of illustrative structural systems and fluid flow-structure interaction systems, in which the fluid is modeled using the Navier-Stokes equations, was calculated. The systems considered present both stable and unstable behaviors, and their LCEs and local stability indicators were computed using the proposed procedures. The stability of the complex behaviors exhibited by the problems considered was properly captured by both approaches, confirming the validity of the procedures proposed in this work. / by Sandra Rugonyi. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 383-389). / Vortex Induced Vibrations (VIV) of a flexibly mounted rigid cylinder placed in a flow is a canonical problem of fluid-structure interactions and the study of VIV and the resulting material fatigue is particularly important in design for ultra-deep water oil exploration and development. Long cylindrical structures such as risers and mooring lines can be outfitted with VIV-cancellation systems or subjected to aggressive replacement schedules. However, the cost of these measures can be high. Effective VIV and fatigue prediction programs offer the potential for design and replacement schedule optimization. For a long time, researchers believed that these oscillations were mainly in the crossflow direction (a direction perpendicular to the flow direction) and were purely harmonic ones with only one major frequency close to the Strouhal frequency (a dimensionless representative of the frequency of vortex shedding). The study of inline motion of long flexible cylinders caused by VIV has been long neglected due to its small amplitude compared to the cross-flow response amplitude. However, it is shown that the inline motion has a major impact on fatigue life due to its higher frequency (second harmonic). More importantly, it triggers a third harmonic stress component in the crossflow direction along with a chaotic frequency stress component. In this thesis, first the impact of the higher harmonic components and the chaotic response on the resulting fatigue damage of the flexible cylinders was first systematically assessed. Statistical methods were employed to generate a stress time series based on the power spectrum density (PSD) plots similar to those of the experimental signals. It is shown that the high harmonic signal has a significant influence on the fatigue life, while the wide-band chaotic components have limited effects. Thus, prediction of high harmonic forces through coupled inline-crossflow VIV modeling is the key to the problem. Thereafter, the semi-empirical crossflow VIV prediction model was revisited. Prediction results from VIVA - a VIV response prediction program widely used in the offshore industry - were improved through the systematical adjustment of hydrodynamics coefficients based on field and laboratory experiments. The results were benchmarked with controlled laboratory experiments, including the Norwegian Deepwater Programme (NDP) Riser High Mode VIV test, the Chaplin experiment, the ExxonMobil 10m long riser model datasets, and field experiments including the Miami experiment (MIT-DeepStar datasets) and the BP bare full-scale riser datasets in the Gulf of Mexico. Afterwards, pure inline VIV was studied using both experiments and a semiempirical model. A pure inline VIV hydrodynamic force coefficients database was constructed from forced motion experiments in the towing tank. A spring-danper rigid cylinder VIV model was created and the pure inline hydrodynamic database was employed to calculate the fluid forces to the system. The motion predicted by the model was compared with free vibration experiments. Additionally, an inline response prediction module was introduced to VIVA to predict the inline response of flexible cylinders. The results were benchmarked with the Norwegian Deepwater Programme (NDP) Riser High Mode VIV test data. Finally, the coupled Inline-crossflow VIV problem was systematically addressed. Extensive forced inline-crossflow experiments were designed and carried out in the MIT towing tank, which provided hydrodynamic coefficient databases for VIV prediction models. The constructed 2D VIV hydrodynamics coefficients database 'ZDMiT' was thoroughly studied and is expected to be useful for semi-empirical programs predicting coupled inline-crossflow VIV in the field. Additionally, a rigid cylinder VIV prediction model was developed to estimate the coupled inline-crossflow VIV response and the high harmonic VIV forces. / by Haining Zheng. / Ph. D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2000. / Includes bibliographical references (p. 68-72). / Sustainable development is not just about technological innovation, but rather about a radical shift in the way society thinks. The environmental effects of our choices and behavior must be internalized. In the context of product development, this internalization should occur in early product development under the guidance of an environmental expert. During early product design phases there may be numerous concepts with significant differences, detailed information is scarce, and decisions must be made quickly. The overhead in developing parametric life-cycle assessment (LCA) models for a diverse range of concepts, and the lack of detailed information make the integration of environmental expertise through traditional LCA models impractical. Therefore, a new approach was developed to incorporate analytically based environmental assessment in early design stages. Product descriptors are the communication interface between environmental experts and designers for this new model, called a learning surrogate LCA. Product descriptors are a set of keywords both understood by designers in relation to a preliminary product, and meaningful in an approximate environmental impact assessment of a product. This thesis develops a set of product concept descriptors for use in environmental assessment. The chosen descriptor set was measurable by designers in conceptual design, and produced reasonable results when used to predict environmental impacts using an approximate model. Tests within the DOME integrated modeling environment have shown it is possible to predict the life-cycle energy consumption of a product. There is also a basis for the method to be used in predicting solid material, greenhouse effect, ozone layer depletion, acidification, eutrophication, winter smog, and summer smog. / by Julie L. Eisenhard. / S.M.

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 75-80). / Low frequency insulator based dielectrophoresis (iDEP) is a promising technique to study cell surface dielectric properties. To date, iDEP has been exploited to distinguish, characterize, and manipulate particles and bacteria based on their size and general cell phenotype (e.g. gram positive vs. gram negative). However, separation of bacteria with diverse surface phenotypes but similar sizes sets a much higher demand on separation sensitivity, necessitating improvement in channel structure design in order to increase the electrical field gradient. In this work, a three dimensional insulator based dielectrophoresis (3DiDEP) microdevice is designed to achieve continuous particle sorting based on their size and, more importantly, on their surface dielectric properties. A 3D constriction is fabricated inside Poly (methyl methacrylate) (PMMA) channels using a micromilling technique. By controlling the channel geometry at the 3D constriction area, a nonuniform electric field with a large intensity gradient perpendicular to the local particle flow direction results in transverse particle deflection, driving particles into different outlet streams. With both simulation and experiments, we show that a diverse array of particles can be distinguished by 3DiDEP. This 3DiDEP sorter can be used in multiple applications in which the surface properties of cells or particles are of special interest. / by Qianru Wang. / S.M.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 103-106). / Gradient index (GRIN) materials offer the most general manipulation over wave fields of light compared to conventional refractive optics, where the light is deflected by the curved surface. The creative way to implementing GRIN optics is to construct a subwavelength structure with the electromagnetic characteristics that are unavailable via the natural material. This artificial GRIN structure also known as "metamaterial" can be classified into two general categories: film and slab GRIN optics, depending on the propagation direction of light penetrating through or propagating along the metamaterial. In this dissertation, two different purposes of all-dielectric GRIN optics on (1) film: light extraction enhancement of the scintillator; (2) slab: aberration-free focusing using Lüneburg lens, are both investigated. The scintillator made by ceramics like Lutetium Yttrium Orthosilicate (LYSO) possesses higher index of refraction at 1.82 than the surrounding environment, which causes extraction loss due to index mismatching and total internal reflection (TIR) from scintillator to photodetector. A hybrid structure including two-dimensional photonic slab covered by the nanocone structure on the top was devised to recycle the energy loss from TIR and to create an index-matching layer in between. Design parameters of the hybrid structure were optimized by the simulation based on rigorous coupled-wave analysis, and the fabrication of hybrid structure was patterned by nanospheres (for nanocone structure) and laser interference (for photonic slab) lithography, respectively. Reactive ion etching (RIE) facilitated pattern transfer after two separate lithography processes. Finally, the characterization of nanostructured scintillator was performed with the ionizing source. The rest of this research focuses on the implementation of the slab GRIN optics: Nanostructured Lüneburg lens. The Lineburg lens is an aberration-free lens that can perfectly focus light on the opposite edge of the lens area, and such property can be used for light coupling from fiber to waveguide in the Silicon photonics. We designed the nanostructured Lineburg lens on the silicon-on-insulator substrate using effective index of refraction computed by photonic band theory, and the fabrication was carried out by the e-beam lithography and RIE process. The device characterized by near-field scanning optical microscopy exhibited the single focusing behavior under fundamental mode illumination via the intensity map over the lens region. In addition, the bi-foci phenomenon under higher order mode illumination was also revealed in the finite difference time domain simulation, and the ray picture for explaining the bi-foci was also included using Wigner distribution function and Hamiltonian ray-tracings. / by Chih-Hung Hsieh. / Ph. D.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1992. / Includes bibliographical references (leaves 49-50). / by Timothy Sean Quinn. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1995. / Includes bibliographical references (leaves 154-158). / by Christine Dianne Allan. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 143-152). / Wearable ultrasound sensing could enable novel medical diagnostics by facilitating continuous, real-time, and direct measurement of physiological phenomena, such as blood pressure. Currently, ultrasound is not used in wearable health sensing applications because clinical ultrasound systems are expensive, bulky, and require high operating power. Realizing wearable ultrasound therefore requires significant reductions in cost, size, and power consumption. Manufacturing cost is of particular concern because sensors are frequently incorporated into consumer goods, where cost is a key driver of technology adoption. Toward that goal, this thesis explored the first steps toward the opportunity to fabricate low-cost ultrasound transducers by contact printing. Contact printing was selected because it could be scaled for high-throughput manufacturing, and it could be performed at ambient temperature and pressure. For this thesis, a capacitive microscale ultrasound transducer was fabricated by contact printing a gold-parylene composite flexible membrane onto a silicon chip substrate. Significant challenges with the adhesion between the membrane and the chip were overcome during fabrication process development and a high yield process for the contact printing step was developed. The transducer was characterized for electromechanical performance. The first mode resonant frequency of the transducer was 2.2MHz, with a 2MHz bandwidth, placing it in the range of interest for medical ultrasound applications (typically 1-15MHz). These results demonstrate that flexible membrane ultrasound transducers can be fabricated. Furthermore, they illuminate a path toward wearable ultrasound sensing and more broadly, flexible medical devices. / by Megan Johnson Roberts. / Ph. D.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006. / Includes bibliographical references (p. 116-118). / This thesis documents the design of a multi-axis nanopositioner that addresses a need for carbon nanotube (CNT) instrumentation that is capable of multiple modes of mechanical testing. This nanopositioner is a solution to the need to quantify the mechanical properties of CNTs with the appropriate modes of testing, such as simultaneous bending and tensile loading. This information is important as it is required to test and better understand the properties of CNTs before and after they are used in micro/nano-structures. The multi-axis nanopositioner will be integrated as one of the core components in a new CNT instrument that is presented in this thesis. The nanopositioner is a compliant mechanism-based device designed that is to induce precise nanometer-level deformations in CNTs within a scanning electron microscope (SEM). The design presented in this thesis is a 4-axis prototype of a 6-axis version. The 4-axis nanopositioner was able to demonstrate over one micron range of motion in multiple axes with 10 nm resolution and repeatability. The nanopositioner was specifically designed to fit inside an SEM like an ordinary sample. / by Kevin Lin. / S.M.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1988. / Includes bibliographical references. / by Martin Scheidl. / B.S.